Machine-readable hologram cards have recently been proposed for use as financial transaction cards and in other applications in which security is important. Such cards encode alpha and numeric data using multiple diffractive optical elements (DOEs). A major advantage of such encoding is that the data stored in the card cannot be read by the user, or by an unsophisticated counterfeiter.
FIG. 1A is a perspective view of a machine-readable hologram card 1. Such a machine-readable hologram card and a card reader for such a card is described in U.S. Pat. No. 4,204,638 of Laude, and by Tompkin et al. in Low-Density Diffractive Optical Memories for Document Security, 35 OPT. ENG., 2513-2518 (September 1996). In the machine-readable hologram card 1, the diffractive optical element (DOE) 3 is located on the reading surface 5 of the card. In the example shown, the data storage capacity of the card is increased using multiple DOEs arranged along the track 7.
FIG. 1B shows a cross-sectional view of a small part of a machine-readable hologram card 1 taken through part of the DOE 3. The DOE 3 includes the thin plastic substrate 17 having the irregularized diffraction grating 11 formed in its surface 13. The depth of the diffraction grating is exaggerated in the figure to show the contours of the diffraction grating more clearly. The diffraction grating is typically formed by embossing the substrate 17. The surface 13 is covered by the thin reflective layer 15. The reflective layer is normally a thin layer of metal deposited on the surface 13 before or after the surface is embossed. The DOE is affixed to the reading face 5 of the substrate 9 of the card 1 by a suitable adhesive.
FIG. 2 shows a typical card reader for reading the machine-readable hologram card shown in FIGS. 1A and 1B. A collimated light beam from the laser 21 is directed by the mirror 23 onto the DOE 3 on the reading surface 5 of the machine-readable hologram card 1. The DOE generates a hologram image by diffracting the collimated light beam. The irregularities formed in the diffraction grating 11 cause the diffraction grating to generate a number of satellite beams at characteristic angles and with characteristic intensities. A detector is provided in each of the anticipated paths of the main and satellite diffracted light beams to detect the intensity of the diffracted light beam. The detectors 25, 27, 29 and 31 are shown as examples. The electrical output levels from the detectors are analyzed by the electronic circuit 33 to determine whether the card 1 inserted into the card reader generates the correct combination of detector output levels, i.e., the same combination of output levels as that generated by a genuine card. Moreover, the absence or presence of one or more of the detector outputs may be used to represent binary data that can be extracted by the electronic circuit. The data encoded in successive DOEs disposed along the track 7 are read out by transporting the card in the direction indicated by the arrow 35.
Despite the potential for greater security offered by encoding data as DOEs, known machine-readable hologram cards are not impossible to counterfeit. The use of a limited number of discrete detectors requires that the diffraction gratings generate a relatively simple pattern of diffracted light beams. This requires that the diffraction grating itself be relatively simple. A hologram card using diffraction grating-type DOEs can be counterfeited in one of two main ways. The first main way involves cloning the DOEs taken from a valid, genuine card. The genuine DOEs are used as templates to generate masters from which multiple counterfeit DOEs and corresponding hologram cards can be manufactured.
Cloning involves delaminating the DOEs taken from a genuine hologram card to expose the diffraction grating 11 in the surface 13 of the DOE. It is possible to delaminate the conventional DOE mechanically because the adhesion between the reflective layer 15 and both the card substrate 9 and the DOE substrate 17 is relatively weak. Thus, the DOE can be simply pulled off the card substrate to expose the surface 13. Alternatively, the DOE can be delaminated chemically by dissolving the reflective layer using a chemical that does not attack the DOE substrate 17. Because the metal of the reflective layer is chemically very different from the plastic of the substrates of both the card and the DOE, finding a suitable chemical is not difficult. Once the surface 13 has been exposed, known electroplating methods can be used to derive a master from the surface 13. The master can then be used as a stamper to make the DOEs for multiple counterfeit machine-readable hologram cards using conventional embossing techniques.
The second main way of counterfeiting a machine-readable hologram card is to make the master for each DOE from scratch, and then to use a known method to derive from the master a stamper that can then be used to manufacture the DOEs for multiple counterfeit cards using conventional embossing techniques. In conventional machine-readable hologram cards that are designed to be read using the apparatus shown in FIG. 2, the diffraction gratings in the DOEs have topographical features with linear dimensions of several microns. The masters for patterns with such dimensions can be printed with sufficient resolution using high-resolution printers of the type commonly used for typesetting.
A paper by Steve McGrew entitled Countermeasures Against Hologram Counterfeiting, http://www.iea.com/.about.nli/publications/countermeasures.html, suggests that delaminating metallized holograms could be made more difficult by making the holograms "multiply connected." The paper explains that a multiple connectivity means that the hologram is composed of dots or else is punched full of holes. The figure illustrating a "multiply connected" hologram is not published. However, the description indicates that the active areas of a "multiply connected" hologram will still include an aluminum layer. Although multiply connecting the hologram will make the hologram more difficult to counterfeit, retaining the aluminum layer provides an avenue through which the embossed surface can still be accessed. Moreover, the need to pattern the aluminum layer complicates the process of manufacturing such holograms.
Accordingly, there is a need for a machine-readable hologram card with considerably higher security than known machine-readable hologram cards. The DOEs in such a card should generate a more complex hologram image than the DOEs of known hologram cards, and the more complex hologram image should be capable of easy detection. Moreover, such a card should be simple to manufacture but should be very difficult to counterfeit either by cloning or by making a master from scratch. There is also a need for an apparatus capable of reliably reading such a high-security machine-readable hologram card.